The present invention relates to the field of optical communications systems and more specifically to the field of wavelength multiplexer/demultiplexers.
Optical communication systems use optical signals to convey information over an optical transmission medium, typically a waveguiding medium such as optical fiber. The usable transmission capacity of a given optical waveguiding medium can be substantially increased by the use of wavelength division multiplexing (WDM) techniques. WDM is typically used for long-haul transmission systems but can also be useful for small local area networks. WDM is a method for increasing the capacity of an optical transmission medium by simultaneously operating more than one optical signal at different wavelengths over one medium. With WDM, different multiplexed optical signals can be transmitted at different wavelengths, referred to as channel wavelengths, through the same transmission medium.
One method of routing, switching, multiplexing, and demultiplexing optical signals in a WDM system is to utilize an interconnection apparatus having closely spaced input waveguide ports communicating with the input of a circular-shaped slab coupler. The output of the slab coupler communicates with an optical grating comprising a series of optical waveguides, each of the waveguides differing in length by a predetermined fixed amount with respect to its nearest neighbor. The grating waveguides are connected to the input of a second slab coupler, the outputs of which form the output ports of the routing, switching, multiplexing and demultiplexing apparatus.
A plurality of separate and distinct wavelengths each launched into a separate and distinct input port of the apparatus will all combine and appear on a predetermined one of the output ports. In this manner, the apparatus performs a multiplexing function. The same apparatus may also perform a demultiplexing function. In this situation, a plurality of input wavelengths is directed to a predetermined one of the input ports of the apparatus. Each of the input wavelengths is separated from the others and directed to a predetermined one of the output ports of the apparatus. An appropriate selection of input wavelengths also permits routing between any selected input port to any selected output port.
By multiplexing optical signals at a transmission point of a WDM system and demultiplexing the optical signals at a reception point, the capacity of the physical transmission medium is increased. However, along with the increase in capacity, physical network element management capability is required to ensure the stability and reliability of the network and network elements such as optical amplifiers, multiplexers, demultiplexers, wavelength adds, wavelength drops and wavelength cross-connects. Typically, out-of-band signals (optical signals that are not within the wavelengths used to carry data signals within the WDM system) are used to administer information among the network elements (for example, optical amplifiers, multiplexers, demultiplexers, wavelength adds, wavelength drops and wavelength cross-connects) in a conventional WDM system. These out-of-band signals can be, for example, a supervisory signal, for monitoring and assuring reliable network efficiency, or a pilot signal, for maintaining the total power transmission levels in a network and compensating for any loss of power.
A conventional router in a WDM system is used to demultiplex/multiplex the precise WDM wavelengths, with WDM channels typically spaced apart in 100 GHz or 50 GHz in frequency increments. The lasers for the channels are typically stabilized with wavelength drift of the order of 1 GHz. In the example of an out-of-band signal such as a supervisory signal (SUPV), whenever a fault is detected by the supervisory system, various parts of the transmission system switch to standby equipment or other necessary fault recovery techniques. SUPV signals typically are transmitted over service channel fibers and therefore allow the full capacity of the primary network to be used for data transmission. This supervisory control provides efficient and reliable network management. In accordance, the International Telecommunications Union (ITU) has proposed the use of supervisory signals along supervisory channel wavelengths for C-band WDM systems, with wavelengths reserved outside the data channel bands. The ITU recommendation for such out-of-band wavelength is 1510 nm (plus or minus 10 nm). Typical stability requirements for SUPV lasers are less than 100 GHz drift. As WDM systems move into L-Band, some proposed wavelengths for SUPV are 1480 or 1624 nm with 100 GHz drift.
A conventional technique to support SUPV or any out-of-band signal in a WDM system is to use two separate filters. One filter, typically an optical bandpass filter, is necessary for adding the SUPV signal into the transmission medium such as optical fiber and another filter is necessary for eliminating the SUPV signals before the WDM signals are transmitted into the network elements. However, extra power losses from the additional SUPV filters can impose a power penalty on the WDM system and consequently result in transmission distance reduction in existing WDM systems.
Accordingly, there is a need for more efficient power transmission between the input and output ports of a WDM system. There is a further need for supporting SUPV and other out-of-band signals in a WDM system without the corresponding power loss and transmission distance reduction found in conventional WDM systems.
The present invention is an integrated wavelength router which incorporates out-of-band optical signals that can be used in network management for increasing transmission distance or span length. Elimination of bandpass filters, otherwise used to remove out-of-band signals in conventional wavelength routers, allows for additional insertion power losses to be reduced and consequently can increase the span length of the WDM system.
In an exemplary embodiment of the invention, planar waveguide routers (WGR) which can incorporate out-of-band signals such as the supervisory signal by proper control of the free-spectral-range (FSR) periodic property of the WGR device are demonstrated. The incorporated out-of-band signal is combined into the same input ports on the WGR as that for WDM data signals at the transmission point of the WDM system and another output port on the WGR at the reception point of the WDM system. The demonstrated WGR can be used at any point in the WDM system as a network element (for example, multiplexer, demultiplexer, wavelength add, wavelength drop, wavelength cross-connect, etc.).